The present invention pertains to halogen-containing cured or self-cured compositions and their methods of preparation. More specifically, a halogen-containing epoxy compositions can be formed by mixing an epoxy resin, an amine curing agent and a halogenated amine. The resultant halogen-containing compositions have improved tribological properties, namely reduction of friction and wear.
Epoxy resins represent an important class of polymers primarily due to their versatility. High degree of crosslinking and the nature of the interchain bonds give cured epoxies many desirable characteristics. These characteristics include excellent adhesion to many substrates, high strength, chemical resistance, fatigue resistance, corrosion resistance and electrical resistance. In addition, processing is simplified by the low shrinkage and lack of volatile by-products. Properties of the cured epoxies such as mechanical strength or electrical resistance can be optimized by appropriate selection of the epoxy monomer and the curing agent or catalyst. Because of the ease of application and desirable properties, epoxies are widely used for coatings, corrosion protectants, electronic encapsulants, fiber optic sheathing, flooring and adhesives.
Epoxy compounds were first synthesized as early as 1891; however, commercialization did not come about for the next 50 years. Two independent researchers, developing separate applications, synthesized the first commercial epoxy resins. Pierre Castan of de Trey Frères in Switzerland, while developing dental restoration materials, discovered the reaction of diglycidylether of bisphenol-A (DGEBA) with phthalic anhydride. The patents were assigned to Ciba AG of Basel, Switzerland (now Ciba-Geigy) in 1942. At the same time, Sylvan Greenlee at DeVoe and Raynolds (later Celanese Chemical Company, and subsequently Hoechst-Celanese) in America, while developing surface coatings, discovered another DGEBA resin, which differed only in molecular weight. Greenlee's first of many patents was granted in 1948. These DGEBA resins and subsequent derivatives have, and continue to be, the largest product in the epoxy market, primarily in the surface coatings industry for which it was developed. The characteristics which Greenlee and Castan sought and found in DGEBA, including adhesion, hardness, inertness and thermal resistance, are responsible for its popularity. Many other monomers and polymers have been subsequently epoxidized to increase the desirable properties of DGEBA and to develop special properties such as high electrical resistance and thermal stability.
Epoxies are characterized by the presence of one or more epoxide functional groups on or in the polymer chain. The epoxide group is planar, with a three-membered ring composed of one oxygen and two carbon atoms. Due to the high ring strain, similar to that in cyclopropane, the group is very reactive.
The ring-opening polymerization and crosslinking in epoxy resins can be of two general types, catalyzed homopolymerization or bridging reactions which incorporate a coreactive crosslinking agent into the network. Homopolymerization, or reactions between epoxy chains, involve elimination reactions on the oxygen atom of the epoxide group using acid or base catalysts, often activated by radiation. The incorporation, or bridging reaction, involves nucleophilic attack on one of the epoxide carbons by an amine or an anhydride compound. An obvious and important difference in the result of the two different curing methods is that in homopolymerization the network is only composed of the cross-linked epoxy monomers, whereas in the bridging reaction the network is composed of a copolymer of both epoxy monomers and a curing agent. Therefore in a bridging reaction the network properties are a function of two components, which allows modifications to be incorporated in either component.
Epoxies and curing agents have been chemically modified for a variety of special purposes, with recent attention given to the addition of fluorine functional groups to increase electrical resistance and dielectric constant as well as for improved tribological properties, namely reduction of friction and wear. While significant work has been done in fluorinating epoxy resins or epoxidizing fluoropolymers, the costs are typically prohibitively high. Researchers continue to develop economically viable epoxy with the friction-reducing fluoro groups bonded into a wear-resistant epoxy network. A preferred embodiment of the current invention utilizes a commercially available fluorinated amine as a curing agent.
The physical properties of uncured epoxy resins vary widely. As with any polymer, the viscosity of the monomers or prepolymers depend on both the molecular weight and the molecular structure. A simple example is DGEBA, as shown in FIG. 1. Higher linear molecular weight monomers, i.e. those with higher values of n, exhibit higher viscosities. In addition, molecular structure and types of bonds will greatly affect the viscosity of the resin. Since epoxies are almost always used with catalysts, crosslinking agents, accelerators and various other additives, viscosity effects like plasticization should be considered.
Epoxy curing involves two phenomena, polymerization and crosslinking. Although each phenomenon is complicated and the two are in competition during the overall curing process, generalizations and simplified models can be made. During the initial stage of curing, polymerization is favored because in the case of curing agents, primary reactions are more reactive than secondary ones. Also, the terminal epoxide reactivity already mentioned plays a role. In most cases the polymerization is an addition reaction, and thus follows a rate equation for addition polymerization described later. The molecular weight of the growing polymer increases until the molecular weight approaches infinity, so that all monomers are connected by at least one bond and a network is formed. At this point, called the gel point, the polymer possesses high molecular weight and few crosslinks, and thus behaves much like a very high molecular weight thermoplastic. From the gel point, crosslinking becomes the dominant phenomenon due to the lack of free monomers. Crosslinking involves interchain bonding of intrachain reactive sites, either intrachain epoxides or secondary sites on coreactive agents.
Although crosslinking is a different phenomenon, the rate of chemical conversion of the epoxide groups is unaffected in most epoxy systems. The crosslinking reactions produce a growing network and reduce the mobility of the chain segments. The growth of the network results in mechanical and thermal stabilization of the structure, resulting in increasing modulus and glass transition. At a certain high degree of crosslinking, the increasing molecular weight of the structure exceeds the molecular weight which is thermodynamically stable as a rubber, and the material transforms into a glass, a process called vitrification. In a glassy state, the mobility of reactants is severely restricted, reducing the rate of the reaction to a diffusion-controlled reaction, which is much slower. Further conversion is still possible, however, the rate is much slower since the process relies on diffusion rather than mobility to bring the reactants together. When the crosslinking reaction exhausts all the reactive sites available, the resulting structure is hard (high modulus) and insoluble due to a high degree of interchain bonding.
The system consists of a DGEBA epoxy, as shown in FIG. 1, is mixed with an aliphatic amine curing agent, as shown in FIG. 2. This system is similar to many of the common commercial epoxy-based adhesives in which the epoxy resin is mixed with an amine curing agent by volume. This system was characterized after mixing the two components per the manufacturer's recommendations, but was also used in a fluorination modification procedures. Specific amounts of a fluorinated amine curing agent were substituted for some of the aliphatic amine. Previous studies conducted by W. Brostow, et al., Mat. Res. Innov., 6 (2002) 7, on thermoplastic blends with specialized components demonstrated that small amounts of fluoropolymer additives produced large effects on tribological properties of the epoxy. Therefore, small amounts of fluorinated amines were substituted. The chemical structures of the fluorinated amines used are provided in FIG. 3. The fluorination described herein provides an economically feasible method of reducing the friction on the cured epoxy surface. Most previous attempts have focused on synthetically fluorinating the epoxy chain, which is both complicated and costly.